Apparatus for processing refuse to produce steel-making scrap

ABSTRACT

Refuse scrap from incinerated refuse or nonincinerated refuse is dried and precleaned to a metal content in excess of 70% and is then comminuted and abraded in a hammer shredder with the comminuted material being subjected to air separation and magnetic separation to yield a metal product containing at least 70% metal and with a density of at least 1 ton per m 3  for direct use as any other iron scrap in a steel-making plant.

This application is a division of application Ser. No. 544,403 filedOct. 21, 1983, now abandoned.

FIELD OF THE INVENTION

Our present invention relates to a method of and to an apparatus for thetreatment of refuse scrap, i.e. the treatment of the primarily metallicproduct which can be extracted in a recovery operation from incineratedor nonincinerated refuse and specifically municipal refuse.

BACKGROUND OF THE INVENTION

In recent years, with increasing concern for the environment, methodsand systems have been devised to reduce the amount of refuse which mustbe deposited in landfills or dumped into the oceans.

Municipal refuse and garbage, for example, is increasingly processed torecover valuable components therefrom with the remainder being burned orotherwise treated so that only a reduced volume of waste need bepermanently disposed of as ash or as sanitary fill. Waste and refuseprocessing plants recover a metallic component from the unburnt waste,this product being referred to as unburnt refuse scrap. In othersystems, the refuse or garbage may be incinerated and the burnt productsubjected to separation of metals, thereby yielding burnt refuse scrap.

Since the metal separation in the case of the processing of incineratorproducts is effected by magnetic means, the burnt refuse scrap cancontain high proportions of iron.

Consequently, the term "refuse scrap" or terms of similar significanceare used herein to refer to the product having a high content in metalsand generally a comparatively high proportion of ferrous metal, obtainedby the processing of garbage and refuse and especially municipal garbageand refuse. The distinction between an incinerator product and a productobtained without incinerator processing can be discerned in thedistinctions made between burnt and unburnt refuse scrap below.

Burnt and unburnt refuse scrap contain a significant proportion ofnonmetallic components. As noted, the separation of iron from thenonferrous components in a refuse generally is effected in garbage andwaste processing installations by magnetic separation techniques. Theiron-containing scrap which thus results and of which more than 90% isobtained from the burnt refuse scrap, i.e. the incinerator metal scrap,has hitherto been utilized as an additive to blast furnaces. Howeverproblems have been encountered with this method of disposing orutilizing the metal scrap.

For example, by comparison with other iron sources in the form of otherscraps, the refuse scrap has a low metal content (about 60 to 70%) and aproportionately higher amount of nonmetallic components. Such metalcontent is between 20 and 30% less than that of other scrap sources formetallurgical processes.

The refuse scrap in the past has also had a low apparent specificgravity (piled weight or bulk density) generally ranging between 0.3 to0.4 metric ton per m³. This means that large volumes of material have tobe handled at high cost.

Finally, the chemical composition or analysis of the refuse scrap,especially with respect to sulfur (up to 0.1% by weight), tin (up toabout 0.6% by weight) and copper, chromium and nickel, complicatedmetallurgical use of the refuse scrap in the aforedescribed manner.

Another disadvantage of refuse scrap, apart from its comparatively lowmetal content, is its comparatively high content of slag formers ofslagging components. These accompanying slag formers increase the amountof slag which is produced and which thus must be handled and also aredetrimental to the metallurgical process since the slag formers areoverproportional by comparison to the metal added to such processes.

Indeed, the reduced basicity in terms of the ratio of calcium oxide tosilicon dioxide (CaO/SiO₂), of the slag materials entailed forincinerator refuse slag additional lime which had to be added in thesteel-making process. If additional lime is not added, the lower finalbasicity of the slag results in a poor sulfur distribution between themetal bath and the slag so that the steel which results may have anexcessive sulfur content.

Furthermore, it has been found that the unknown and variable iron oxidecontent of the refuse scrap can pose a significant problem. For example,it is difficult to establish the required steel tapping temperature ifthe amount of iron oxide introduced with the scrap is unknown therebyleading to excessive numbers of overcooled melts which, for effectivecasting must be afterblown in extra process steps at increased cost.

Indeed, the output of high-grade steel falls when afterblowing of a meltis required and significant amounts of iron can be lost because ofentrainment of the iron into the slag as a result of afterblowing.

In part because of the iron oxide introduced and in part of the ironlost in the slag, added quantities of deoxidizing materials may berequired at high cost and finally the larger quantities of deoxidizerswhich may be required tend to lead to steel of reduced purity.

OBJECTS OF THE INVENTION

It is the principal object of the present invention to provide animproved method of processing refuse scrap and especially refuse scrapnow available from waste recovery installations, whereby this scrap maybe utilized more effectively in steel-making processes without thedisadvantages enumerated above.

Another object of the invention is to provide a plant for processingsuch refuse scrap.

SUMMARY OF THE INVENTION

The invention is based upon our surprising discovery that when burnt orunburnt refuse scrap as defined previously, i.e. the scrap obtained fromincinerator residues or from ferrous metal separation from anonincinerated municipal refuse, is subjected to drying and precleaningto a metal content in excess of 70% and the product is then mechanicallyprocessed at least in part by an abrasive or rubbing operation and theresulting comminuted product is then subjected to air classification orsifting and magnetic separation until the metal content is brought to aminimum of 90% and a piled weight or bulk specific gravity of at least 1ton per m³, a product is obtained which can be utilized directly in asteel-making process without increasing the need for deoxidants, withoutcreating an imbalance between metal and scrap and, indeed, which can beused in the same manner as any other iron scrap.

Thus the method of the invention comprises the steps of: drying andprecleaning the refuse scrap (preferably in a rotary drum but also ifdesired in a drying cascade or vibratory sieve device)to obtain aproduct whose metal content is at least 70% by weight;

mechanically treating the resulting product to subject it to mechanicalcomminution and mechanical surface abrading or rubbing to reduce theparticle size of this product and effectively rub the surfaces of theresulting particles free from iron oxide and other contaminants whichmay pose a problem in the final metallurgical handling of the product;

subjecting the thus comminuted and frictionally treated product to airseparation, thereby removing light particles of abraded material fromwhich metals may be recovered and leaving a more dense product;

subjecting this more dense product to magnetic separation to segregatean iron-containing component from a nonferrous component (from whichmetals may also be recovered); and

feeding the iron-containing component, which should have a metal contentof at least 90% by weight and a bulk specific gravity of at least 1 tonper m³, to a steel-making plant.

The product thus introduced to the steel-making plant has a high degreeof purity and a sufficient specific gravity such that it is easilyhandled. The most surprising thing, however, is that regardless of thevarying degree of oxide content of the starting products, the finalproduct transferred to the steel-making plant is uniform and practicallyfree from anything which would give rise to difficulties in subsequentmetallurgical processing or would require special operations such asafterblowing or deoxidants. Apparently the intensive comminution andabrading or frictional treatment of the refuse scrap removesslag-forming components,lacquer or paint layers and oxide layers fromthe surfaces and transforms these contaminants into light weightcomponents which are removed in the air separation or from which theiron component is extracted by the magnetic separation. Paper, plasticsrubber and like contaminants, which may have adhered to or have beenentrained by the metal components, are likewise separated from the ironcomponent.

The high density, high purity product can be charged efficiently intothe steel-making apparatus and the operating costs utilizing the productof the invention are markedly reduced.

In specific terms, a number of improvements can be noted with respect tothe unprocessed incinerator refuse slag.

Firstly, the lime requirements are not increased nor is the amount ofslag increased on use of the product of the invention.

Secondly, because of the use of this product, a higher slag finalbasicity is obtained with improved sulfur distribution between the bathand the slag, the steel having a lower sulfur content as a result.

Thirdly, reproducibility is ensured and hence it is easier to set thetapping temperature.

Fourthly, a greater crude steel output is obtained because afterblowingis not necessary and iron is not entrained into the slag, lessdeoxidizing agents are required and as a result of the reduced amount ofdeoxidizing agents, the steel has higher purity.

Additional advantages are obtained when the use of the product of theinvention is compared with the use of unburnt refuse scrap because ofthe problems introduced when large quantities of entrained papers,plastic and rubber or the like are introduced into the steel-makingfurnace or must be removed at the plant in the quantative way before thefurnace is charged with this scrap.

A critical advantage over most types of earlier scrap obtained with theproduct of the invention is that it can be used directly because of itssmall particle size as a feed for modern giant blast furnaces.

We have found, moreover, that the process of the invention markedlyreduces the tin content in the final product, presumably as a result ofthe surface action during the mechanical treatment. Apparently, the tinlayer is adherent to the surface of other metal particles (in the amountof over 60%) and the mechanical operation eliminates at least a portionof this tin from the surface so that it can be separated out by themagnetic or air separation. Similarly, the mechanical operation followedby the two separation steps has been found to reduce the copper,chromium and nickel contents by more than 50%.

When scrap contains the shredder scrap from the shredding of automobilebodies, it has a high tin content.

The scrap product of the invention is in its physical characteristicsand as to its chemical and tin contents similar to the scrap obtained bythe shredding of vehicle bodies. If the tin level obtained from thereduction in the tin content described is not satisfactory, a furthertin removal operation can be provided. Preferably, the present cleaningof the refuse scrap is effective in a rotary drum, a cascade or via avibration sieve and tends to eliminate moisture and light-weight nonmetallic components. It has been found to be advantageous to follow thisprecleaning by a dust removal operation so that there is noenvironmental contamination by any dust which may be present in therefuse scrap.

According to another feature of the invention, leading to increasedeconomy, the scrap is separated into at least two fractions, namely, thelarge fraction and the fine fraction which are separately treated atleast in part.

The separation of the scrap into fractions provides an early means ofseparating the scrap into two components one of which has a higher tincontent than the other. For example, the component having a higher tincontent can be subjected to a mechanical treatment different from thatutilized for the component with the lesser tin content, thereby reducingthe size of the mechanical comminuter in the abrading unit in the lattercase and reducing operating and capital cost. It is indeed surprisingthat a size classification results in a separation of a component havinga higher tin content from a component having a lower tin content. Thehigher tin component is generally the small size fraction.

The simultaneous drying, precleaning and fractional separation iseffected, according to the invention, in a rotary drum having openingsof a predetermined size and the drying process is effected either byfrictional heating to generate it by rubbing of the particles againsteach other or by introducing heat, e.g. via a drying gas or heating ofthe drum.

The dust removal operation can be carried out before or simultaneouslywith the beginning of the mechanical treatment, e.g. mechanicalcomminutation and rubbing of the scrap.

Since the dust collected may contain iron, the dust can be delivered toa metal residue recovery unit for, for example, chemical separation ofiron and other valuable components.

Especially effective results are obtained, in accordance with theinvention, when the mechanical treatment of scrap is effected until thecomponents subjected to this treatment are in a shiny, bright or cleansurface state. After the separations of the invention this assures asignificant reduction in the sulfur, tin, chromium and nickel content inthe product which is to be fed to the steel-making plant. The heatgenerated by the mechanical comminution and operation contributes to thebreak-down or removal of any lacquer and paint coatings.

The mechanical treatment of the refuse scrap until a bright surface isformed on the components treated has been found to remove, in the caseof incinerator scrap, the extremely adherent oxide layer as well as thelarge proportions of sulfur which are predelivered. The sulfur contentof such scrap is at least twice that of commercial scrap and has createdsignificant problems with respect to desulfurization in the steel-makingplant. The added expense of such desulfurization is avoided with thepresent invention.

In conjunction with the mechanical treatment and/or downstream thereof,it has been found to be advantageous to separate a large component fromthe small component and recycle the large component through themechanical treatment stage thereby increasing the bulk density of theproduct.

According to yet another feature of the invention the abraded materialwhich is separated from the metallic component by the air separationstep is also supplied to a metal recovery unit because it may containcomparatively large quantities of iron and tin and even copper and zinc.This increases the economic efficiency of this system.

From the magnetic separation step a residue is obtained from whichcopper and zinc can be recovered together with small amounts of iron.The latter is only the iron which is adherent to the other metals andthus is not retained by the magnets.

A further feature of the invention provides a detinning stage followingthe magnetic separation stage to remove any further undesirable amountsof tin. With the aid of this latter stage, a high purity scrap can beobtained which can be utilized in the production of steel which musthave a low steel content.

Another advantage of the invention is that it is not necessary to modifyprocess stages or even the apparatus depending on the type of refusescrap which is processed and indeed both burnt and unburnt refuse scrapcan be used and use can be made of mixtures of them without concern forthe initial differences in composition although it is preferable toprocess them separately.

As a consequence, an apparatus for carrying out the invention can be fedcontinuously with the refuse scrap regardless of size and hence thecharging of the apparatus can be effected by a conveyor belt or thelike.

According to the apparatus concepts of the invention, the intake for themechanical treatment device, the rotor thereof and/or the discharge sideof the device can be provided with a dust removal unit. The dust removalunit can also be provided for or can serve as the air-separating deviceand it is advantageous to provide the air-separating and/or dust removaldevice with an external air source or tube.

Downstream of the mechanical treating device, a separating grate can beprovided which can have openings in the form of regular triangles orsquares with individual cross sectional areas of about 50 cm² and it hasbeen found to be advantageous to utilize as a mechanical comminuting andabrading device a shredder of the hammermill type whose rotor isoperated with a power consumption of 60 to 70% of the maximum power orload capacity of the machine.

Test results are obtained with a 1000 P.S. shredder which has 20% morehammers than the conventional automobile body shredder, is operated witha power consumption of 60 to 70 amperes and, in the best mode embodimentof the invention, has fourteen hammers mounted on six shafts of themachine. The first, third, fourth and sixth shafts can each have twodiametrically opposite hammers while the second and fifth shafts caneach have three angularly equispaced hammers. The weight of each hammershould be about 25% less than the weight of the hammer of an automobileshredder and each hammer should have at its free end a circular armsurface whose center of curvature corresponds to the pivot at which thehammer is secured to the shaft.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription reference being made to the accompanying drawing in which:

FIG. 1 is process flow diagram for the invention;

FIG. 2 is a diagram illustrating the connection of the dust removaldevice in an apparatus for carrying out the invention;

FIG. 3 is an elevational view of the lay-out of the hammers of a rotor;

FIG. 4 is a table showing the number of hammers provided on each rotorshaft;

FIG. 5 is an elevational view of the rotor;

FIG. 6 is a plan view of a portion of a grate downstream of themechanical treatment device; and

FIG. 7 is a view similar to FIG. 6 showing another embodiment of thegrate.

SPECIFIC DESCRIPTION

As will be apparent from FIG. 1, either incinerator or burnt refusescrap or unburnt refuse scrap may be fed continuously at 101 and 102,respectively, in any proportions, via continuous conveyor belts to adrying and precleaning drum represented by the first stage 100 in FIG. 1and by the drying drum unit 1 and the conveyor 30 feeding it in FIG. 2.This unit is provided with dust removal facilities as represented at 103and, simultaneously with the removal of the dust-free product which hasa minimum metal content of 70%, the product can be separated intofractions as represented by the double arrows leaving the separationstage 104. The separation at 104 can be a grading by size utilizing agrate passing particles of the smaller size and retaining particles ofthe larger size.

After grading at 104, or during such grading, another dust removal stage105 can be provided.

Each of the two fractions is then submitted to the mechanical treatmentat 200 in a shredding hammermill as will be described in greater detailhereinafter, the shredding hammermill abrading the comminuted particlesuntil the surfaces thereof are shining.

As the material passes continuously from the hammermill it can pass overa grate which will also be described in connection with FIGS. 6 and 7 sothat large components can be separated at 201 and recycled at 202 to thehammermill. During operation of the hammermill dust removal, asrepresented at 203, covers particulates which may be subjected to metalresidue recovery at 204, thereby separating iron dust if desired.

Following the mechanical treatment, the mass is subjected to airseparation at 300, i.e. air is blown through the comminuted product fromthe hammermill, with the particulates being fed at 301 to a metalrecovery stage 302 where iron and tin can be separated out.

From the air separately stage, the two components are separatedlysubjected to magnetic separation at 400 with the magneticallyattractable product, containing a minimum of 90% metal and having a bulkdensity of at least one ton per m³, being subjected to detinning at astage 500 by premetallurgical processes, if desired, before the productis fed at 600 to the steel plant.

The nonmagnetic residue at 401 is subjected to recovery for copper, zincand adherent iron.

In FIG. 1, the stages illustrated in broken lines are optional butpreferred to the best mode embodiment whereas the stages illustrated insolid lines are essential to the invention.

As can be seen from FIG. 2, the drying and precleaning drum 1, theshredder 2 and the discharge part of the shredder represented at 8 caneach be provided with a dust separator or dust recovery unit as has beenrepresented at 3 or a common dust separator can be connected to theseelements as represented by the lines 4, 5 and 7.

The duct 7 can also be provided with a separate air inlet 9 which canreduce the suction force downstream of the shredder, where the suctionforce functions as an air separator, so that a minimum of iron isentrained from the comminuted product.

As can be seen from FIG. 3, the rotor 10 of the shredder can have sixshafts represented by the numerals I through VI and can be driven in thedirection of arrow 12. Each of these shafts can pivotally carry aplurality of hammers 11 and each of the hammers 11 (FIG. 5) can have afree end of circular arc configuration as shown at 13 while beingpivotally mounted at 14 at the center of curvature of the surface 13.

As can be seen from the table of FIG. 4, the six shafts I through VI canhave hammers A through L so that 14 hammers can be provided in all with3 hammers being provided in the shafts II and V while the other hammersare provided in diametrically opposite paths on each of the othershafts.

Downstream of the hammermill, the scrap can pass over grates 20 and 21as shown in FIGS. 6 and 7 which can be provided with regular triangularopenings 22 or square openings 23 with cross sectional areas of about 50cm² so that the retained larger pieces can be recycled to the mill.

We claim:
 1. An apparatus for processing refuse metal scrap to produce aproduct suitable for use as iron scrap in steel making, the apparatuscomprising:(a) drying and precleaning means for drying and precleaningsaid refuse metal scrap sufficiently to form a product having a metalcontent of at least 70%, said drying and precleaning device comprisingone of a rotary drum, a cascade sieve device, and a vibration sievedevice; (b) mechanical treatment means receiving said product from saiddrying and precleaning means and including at least one hammer millshredder operating at about 60% to 70% maximum capacity than anautomobile body shredder to comminute the product into particles and torub the particles together to clean material from surfaces thereof, theshredder having a rotor provided with only six shafts, two of saidshafts each having only three hammers thereon, and the other four shaftseach having only two hammers thereon, with the two hammers on a first ofsaid other four shafts being disposed diametrically opposite the twohammers on a second of said other four shafts, and the two hammers on athird of said other four shafts being disposed diametrically oppositethe two hammers on a fourth of said other four shafts, each of thehammers having a free end provided with a circular arc surface that iscentered substantially about an axis at which the respective hammer isswingably mounted on its respective shaft; (c) air separator meansdownstream of said mechanical treatment means for separating saidparticles into a light component and a heavy component; (d) magneticseparator means downstream of said air separator means for magneticallyseparating from said heavy component a magnetically interactablefraction having a minimum metal content of 90% and a bulk density of atleast 1.0 ton per m³ ; and (e) a dust removal unit connected to saidmechanical treatment means.
 2. The apparatus defined in claim 1 whereinsaid air separator means forms part of said dust removal unit, and saidunit is also connected to said drying and precleaning means and isprovided with an external air inlet for reducing suction at said airseparator means.
 3. The apparatus defined in claim 2, further comprisinga grate downstream of said mechanical treatment means formed withopenings having configurations of regular polygons with areas of about50 cm².
 4. The apparatus defined in claim 1, further comprising meansfor recycling large pieces of said particles to said shredder.